To realize a long-distance and large-capacity communication system in the next generation, the technology of generating a transmission signal using digital signal processing in an optical transmitter has been studied and developed. For example, a desired optical signal waveform of a dispersion pre-equalized signal, a modulated signal, etc. may be generated using the digital signal processing.
FIG. 1 illustrates an example of an optical transmitter. The optical transmitter illustrated in FIG. 1 includes a light source (LD) 11 and an optical modulator 12. The optical modulator 12 is a Mach-Zehnder LN modulator, and has I and Q arms. Furthermore, the optical modulator 12 has a phase shifter for providing a phase difference π/2 between the I and Q arms.
The continuous wave (CW) light generated by the light source 11 is branched by an optical splitter, and guided to the I and Q arms of the optical modulator 12. Data signals I and Q are provided respectively for the I and Q arms of the optical modulator 12. The amplitude of the data signals I and Q is, for example, 2Vπ. Vπ is a voltage (that is, a half-wave voltage) corresponding to a half period of the optical intensity/drive voltage characteristics. In the I arm, the continuous wave light is modulated by the data signal I to generate an I arm modulated optical signal. Similarly, in the Q arm, the continuous wave light is modulated by the data signal Q to generate a Q arm modulated optical signal. Then a QPSK modulated optical signal is generated by combining the I arm modulated optical signal and the Q arm modulated optical signal.
To generate a high quality optical signal in the above-mentioned optical transmitter, the respective bias voltages of the I and Q arms are appropriately controlled. Thus, the optical transmitter further includes a controller 13, a photo detector (PD) 14, and a detector 15 to control the bias voltage of the optical modulator 12.
The controller 13 superimposes a low frequency signal on the bias voltage of the optical modulator 12. Hereafter, f0 refers to the frequency of the low frequency signal. The modulated optical signal output from the optical modulator 12 includes the frequency component (that is, f0 component) of the low frequency signal. The photo detector 14 converts the modulated optical signal output from the optical modulator 12 into an electric signal. The detector 15 detects the intensity and the phase of the f0 component included in the modulated optical signal based on the electric signal generated by the photo detector 14. Then, the controller 13 performs the feedback control on the bias voltage of the I and Q arms so that the f0 component included in the modulated optical signal may approach zero. As a result, the bias voltages of the I and Q arms are optimized to generate a high quality optical signal. Note that the above-mentioned feedback control may be referred to as automatic bias control (ABC).
The method of controlling the bias of the optical modulator using a low frequency signal in the optical transmitter is described in, for example, Japanese Laid-open Patent Publication No. 2000-162563.
The amplitude of the drive signals (data signals I and Q in FIG. 1) of an optical transmitter may be changed by temperature or aging. However, the change of the amplitude of a drive signal by temperature or aging is small. Therefore, in the conventional optical transmitter, the amplitude of a drive signal is substantially constant during the operation of a communication system.
However, in the optical transmitter which generates a transmission signal using the digital signal processing, the modulation format and/or the amount of pre-equalization may be changed during the operation of a communication system. When the modulation format and/or the amount of pre-equalization is changed, the amplitude of a drive signal of optical modulation may be changed.
For example, FIG. 2A illustrates the waveform of a drive signal when the optical transmitter performs QPSK modulation, and FIG. 2B illustrates the waveform of a drive signal when the optical transmitter performs 16QAM modulation. In this example, the amplitude of a drive signal is about 2Vπ for QPSK. The amplitude of a drive signal is about 0.6Vπ for 16QAM. As illustrated, when the modulation format is changed, the amplitude of the drive signal is also changed.
FIG. 2C illustrates the waveform of a drive signal when the optical transmitter performs the QPSK modulation, and performs pre-equalization. In this case, the amplitude of the drive signal is smaller than Vπ. The pre-equalization is realized by applying a distortion to a signal waveform in a transmitter so that the chromatic dispersion of an optical transmission line between a transmitter and a receiver may be compensated. Furthermore, the pre-equalization may be realized by the digital signal processing.
As described above, with the recent or future optical transmitter, the driving condition (the amplitude of a drive signal in the example above) of an optical modulator may be greatly changed depending on the change of a modulation format etc. If the driving condition is changed, there may be the case in which the bias of an optical modulator is not appropriately controlled, and an optical transmitter does not generate a high quality optical signal.